Conventional X-ray inspection systems that are typically used to inspect the contents of objects, such as packages and containers used in the shipment of cargo among sea, land and air ports, include either a "fan-beam" X-ray system employing a "line-of-detectors" X-radiation detector or a "flying-spot" X-ray system, the latter of which is shown in FIG. 1. The flying-spot system 10 typically comprises an X-ray source 12 located at the center of a rotating wheel collimator 14 having a plurality of apertures 15 disposed therein. As the collimator rotates about the source, a cone of X rays is collimated into a pencil-beam by the rotating apertures. A fixed cross-slit collimator 16 may be situated between the rotating wheel collimator and an object 17 to further define the pencil-beam in one dimension. A line of X rays is then formed that sweeps across and through the object, while an X-radiation detector 18 intercepts those X rays exiting the object. The remaining portions of the image are acquired as the object moves past the detector.
The X-ray source 12 used in this conventional pencil-beam system is typically configured to continuously emit X rays in a "steady-state" manner. Typical pulsed X-ray sources cannot be used in this system because they emit X rays in short bursts, followed by periods of quiescence, which are too long to allow rapid image formation. Specifically, the conventional pencil-beam system 10 requires numerous, i.e., 500 to 1000, samples during each sweep of the X-ray beam to acquire sufficient picture elements (pixels) for resolving an image in a brief span of time so as to be useful. The latency between X-ray bursts generated by a pulsed source compels a slow rotation of the rotating wheel collimator 14, thereby resulting in an unacceptably long inspection time for objects. Accordingly, conventional pencil-beam systems are forced to use steady-state X-ray sources.
However, the X rays generated by the steady-state source must be sampled by the detector continuously during the entire time that the pixels are traversed in order to acquire useful data signals. The detector 18 typically includes photomultiplier tubes encompassing scintillating material. When the X rays strike the scintillating material, visible light photons are produced in the material and detected by the photomultiplier tubes which, in turn, generate the data signals needed to form the images. In addition to these light photons, a plurality of delayed light photons are generated that subsist for a time comparable to the time required to sweep the beam across portions of the objects. This source of noise, called "afterglow", adversely affects the signal-to-noise ratio of the conventional X-ray inspection system 10 and is, thus, a limiting factor in the efficiency of that system.
Accordingly, the present invention is directed to providing an improved method and apparatus for reducing afterglow noise in X-ray inspection systems.